1. Field of the Invention
This invention relates to a method for stripping a photo resist from a layer made of an aluminum alloy, such as aluminum-copper, formed on a semiconductor substrate.
2. Description of the Related Art
Aluminum or aluminum-silicon alloy containing several percent of silicon have been generally used as interconnection layers on semiconductor devices. However, it is also well known that such interconnection layers may be broken down by electro migration particularly in semiconductor devices where a very fine interconnection is required, such as 4 .mu.m for a high speed ECL (Emitter Coupled Logic) or 1 .mu.m for MOS (Metal Oxide Semiconductor), to achieve a greater density of integration and accordingly higher density flow of current therethrough is required. In order to prevent such electro migration of interconnection layers, aluminum-copper alloys containing 2 to 4% copper have come to be used, particularly where high current-density is involved, such as in bipolar devices or a high-speed logic circuits. However, aluminum-copper alloys present the problem the presence therein of residual chlorine components
When an aluminum or aluminum alloy layer is plasma dry-etched using chlorinated gases, such as gaseous mixtures containing chlorine (referred to hereinafter as Cl.sub.2), silicon tetrachloride (referred to hereinafter as SiCl.sub.4) or boron tri-chloride (BCl.sub.3), the mechanism of the undesirable reaction resulting from residual chlorine is as follows. EQU Al+Cl*.fwdarw.AlCl.sub.3.Arrow-up bold., Al.sub.2 Cl.sub.6.Arrow-up bold.
where Cl* denotes a chlorine radical produced in the plasma, and .Arrow-up bold. denotes sublimation. Aluminum (Al) reacts with Cl* to produce AlCl.sub.3 or Al.sub.2 Cl.sub.6, which then sublimes, and the aluminum continues to be etched in this manner as long as residual chlorine is present. When the etch-processed substrate is brought out into open air, the AlCl.sub.3, etc. which has sublimed and then has again deposited on a surface of the side wall of the aluminum layer or on the photo resist reacts with the water content in the open air, because the AlC.sub.3 is deliquescent. So, hydrochloric acid (HCl) is produced from the AlCl.sub.3, etc., according to the following formula: EQU AlCl.sub.3 +3 H.sub.2 O.fwdarw.3 HCl+Al(OH).sub.3
Then, the HCl reacts with the Al to produce AlC.sub.3 again, as follows. EQU Al+3 HCl.fwdarw.AlCl.sub.3 +1/2H.sub.2 O
Thus, the reactions are continued repeatedly. In other words, the corrosion of the aluminum layer continues indefinitely. In order to prevent this corrosion, one or a combination of the below-described procedures are carried out for aluminum, aluminum-titanium (Al-Ti) alloys or aluminum-silicon (Al-Si) alloys, after the dry etching process.
(1) Stripping the photo resist without exposing the substrate to open air so as to prevent the chlorine deposition on the substrate from reacting with the water content of the open air.
(2) Drying the substrate with a hot nitrogen gas flow at a temperature as high as 100 to 200.degree. C., and then washing the substrate with water so as to remove the residual chlorine.
(3) Washing the substrate with water, and then baking the substrate in an oxygen atmosphere at a temperature of approximately 350.degree. C. so as to remove the residual chlorine.
(4) Plasma-processing the substrate in a fluorinated gas, such as CF.sub.4, SF.sub.6 or CHF.sub.3, so as to replace the residual chlorine atoms with fluorine atoms produced in the plasma. Thus a firm aluminum fluoride (AlF or AlF.sub.3) layer is formed over the aluminum surface to prevent the chlorine component from reacting with water content in the air.
(5) Plasma-processing the substrate in hydrogen gas, which reacts with the residual chlorine component to produce hydrogen chloride.
By the application of the above-mentioned procedures, residual chlorine component on substrates made of aluminum or aluminum alloys can be removed, and accordingly corrosion can be prevented. However, when etching aluminum alloys such as aluminum-copper (Al-Cu) or aluminum-copper-silicon (Al-Cu-Si), the residual chlorine component is in the form of Cu.sub.x Cl.sub.y or a mixture of Cu, Cl, Al and carbon from the photo resist, and such materials, having much higher sublimation temperatures than aluminum chloride, are difficult to remove. Therefore, such residual chlorine containing components may cause corrosion even when the above-described chlorine-removal procedures are employed.
Therefore, procedures for more effectively removing chlorine residuals have been sought to overcome the above-mentioned problems.